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Skin involvement is common in all forms of histiocytic disorders and is seen at all ages. In general, the histiocytoses can be divided into those involving dendritic cells and those involving the macrophage–monocyte lineage. Langerhans cell histiocytosis is the most common dendritic cell disorder and haemophagocytic lymphohistiocytosis the most common non‐dendritic disorder. Rarer disorders, the non‐Langerhans cell histiocytoses, can be divided into dendritic and non‐dendritic origin and each group further subdivided into those that mainly involve skin but occasionally have a systemic component and those that may include skin but in which the systemic component predominates. This chapter will provide an overview of histiocytic disorders including a brief discussion of possible pathogenetic mechanisms, followed by a more in‐depth discussion of the skin manifestations, pathology and therapy. The similarities and differences between childhood and adult forms of the diseases will be highlighted.
Keywords histiocytoses, dendritic cells, Langerhans cell histiocytosis, haemophagocytic lymphohistiocytosis, non‐Langerhans cell histiocytoses, skin, histiocytic disorders, Erdheim–Chester disease, sinus histiocytosis with massive lymphadenopathy, juvenile xanthogranuloma